The invention relates to superconducting 2-4-7 oxides and their preparation.
Superconductors are materials which below a certain critical temperature T.sub.c have zero resistance to the flow of electrons. Materials such as Ba.sub.x La.sub.z-x CuO.sub.4 and Sr.sub.x La.sub.z-x CuO.sub.4 were reported to have T.sub.c .about.40.degree. K., Chu, C. W. et al., 58 Phys. Rev. Lett. 405 (1987). These compounds have the K.sub.2 NiF.sub.4 -type structure in which copper ions are located within pseudosquare planar arrangements of the oxygen ligands. The nominal copper oxidation state for this structure is between 2+ and 3+.
Another superconducting material, YBa.sub.2 Cu.sub.3 O.sub.x, the so-called "1-2-3" compound, has a T.sub.c .about.90.degree. K.; Wu, M. K. et al., 58 Phys. Rev. Lett. 908 (1987). X-ray and neutron diffraction analyses revealed that this material essentially has a perovskite structure in which yttrium and barium ions are ordered in the c-axis direction and distinctive oxygen defects cause an orthorhombic unit cell with a space group Pmmm, see, e.g., Seigrist, S. et al., 35 Phys. Rev. B. 7137 (1987). This structure has two principal groups of Cu ions. In one group, denoted crystallographically as Cu II, the Cu ions are located within square planar or square pyramid oxygen arrangements which generate infinite [CuO.sub.2 ] planes extending perpendicular to the c-axis on both sides of the yttrium ions. The other Cu ions (Cu I) are located in square oxygen planes, which form [CuO.sub.3 ] chains in the b-axis direction. The superconducting properties of this compound are influenced sensitively by oxygen stoichiometry. The general consensus is that other defects, particularly twins and grain boundaries, play an important role in its electrical and mechanical properties.
Recently, new compounds were discovered in the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems, Maeda, H. et al., 27 Jap. J. Appl. Phys. Lett. (in press) (1988), Sheng, Z. Z. et al, and Hermann, 332 Nature 138 (1988). T.sub.c in these compounds was about 125.degree. K. Structural analyses of these materials revealed several structures and compounds. The difference in T.sub.c between the various compounds can be related to the number of infinite [CuO.sub.2 ] planes perpendicular to the c-axis. The common features between the 1-2-3 phase and these new superconducting oxides indicates that infinite [CuO.sub.2 ] planes and an intermediate copper oxidation state between 2+ and 3+ are mainly responsible for the superconducting properties. These ideas are also consistent with the result for the 1-2-3 compound presented by Xiao et al. in 322 Nature 238 (1988).
A derivative structure of the normal 1-2-3 structure has also been identified. This compound, named the "2-4-8" structure, has a T.sub.c .about.80.degree. K., as reported by Marshall et al., Abstracts of Mater. Res. Soc., Spring 1988 Meeting 197 (1988) in thin films having the composition Y.sub.2 Ba.sub.4 Cu.sub.8 O.sub.x and by Kogure et al., 156 Physica C 35 (1988) in oxidized metallic precursors having the composition Yb.sub.2 Ba.sub.4 Cu.sub.8 O.sub.x. This structure can be described as having periodic insertions of CuO layers in the normal 1-2-3 structure, which causes double [Cu.sub.2 O.sub.4 ] chains in the b-axis direction.